This application is based on Japanese Patent Application No. 2001-183741 filed Jun. 18, 2001 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an inkjet printing apparatus, a printing head, and an ink supplying method and, more particularly, the invention is preferably applied to an inkjet printing apparatus in which ink is intermittently supplied to a printing head for ejecting ink.
2. Descritpion of the Related Art
Inkjet printing apparatuses which form an image on a printing medium by depositing ink to the printing medium using an inkjet printing head include that which forms an image by ejecting ink while moving a printing head relative to a printing medium and that which form an image by ejecting ink while moving a printing medium relative to a fixed printing head conversely.
There are two general types of methods of supplying ink to a printing head in such an inkjet printing apparatus. One is a type in which a supply system is configured such that an amount of ink is always or continuously supplied to a printing head according to the amount of ink ejected (hereinafter referred to as a continuous supply type), and the other is a type in which a printing head is provided with a reservoir (sub-tank or second ink tank) for reserving a predetermined amount of ink and in which a supply system is configured such that ink is supplied to the reservoir from an ink supply source (main tank or first ink tank) at appropriate timing or intermittently (hereinafter referred to as an intermittent supply type).
The continuous supply type is further categorized into two types, for example, when it is used in an inkjet printing apparatus of a type referred to as a serial type in which a printing head is scanned back and forth in predetermined directions relative to a printing medium and in which the printing medium is transported in a direction substantially orthogonal thereto to form an image. One is a type referred to as an on-carriage type in which ink is supplied by integrally or detachably attaching an ink tank to a printing head that is carried and moved back and forth (main scanning) by a carriage. The other is a tube supply type in which an ink tank that is separate from a printing head carried on a carriage is fixedly installed in a part of a printing apparatus other than the printing head and in which the ink tank is connected to the printing head through a flexible tube to supply ink. In some of the latter type, a second ink tank that serves as an intermediate tank between an ink tank and a printing head is mounted on the printing head or the carriage.
When an on-carriage type structure is adopted, there are limits on the project area in a direction perpendicular to the main scanning direction and volume of members that move with a carriage (a printing head and an ink tank undetachably or detachably integrated with the same). Therefore, only an ink tank having a very limited capacity can be used when a small-sized printing apparatus, especially, a portable printing apparatus is to be formed. This results in very frequent replacement of the printing head integral with the ink tank or the ink tank alone, which has been problematic from the viewpoint of operability and running cost. Further, the recent spread of so-called mobile apparatus is remarkable and, for example, ultra-compact inkjet printers have been proposed which can be integrated with notebook type personal computers and digital cameras. It is considered impractical to design such printers in adaptation to the on-carriage method.
When a tube supply type structure is adopted, although members that move with a carriage during main scanning can be made compact to some degree, it is difficult to make the apparatus as a whole compact because a space is required for a tube member to move to follow up the carriage, the tube member coupling a printing head on the carriage and an ink tank located outside the carriage to supply ink. Further, the recent trend is that a carriage is scanned at a high speed to accommodate increases in the speed of printing operations, and resultant severe rocking of a tube that follows the carriage results in changes in the pressure of ink in an ink supply system for the printing head. It is therefore required to provide various complicated pressure buffering mechanisms in order to suppress pressure changes, it has been difficult to achieve a size reduction in this respect too.
On the contrary, in the case of the intermittent supply method that is used for serial type inkjet printing apparatus for example, a relatively small second ink tank and printing head are provided on a carriage; a relatively large first ink tank is provided in a part other than the carriage of the printing apparatus; and a supply system is configured such that ink is supplied from the first ink tank to the second ink tank at appropriate timing. A structure is also employed in which the ink supply system between the first and second ink tanks is spatially separated or the ink channel is blocked with a valve during main scanning to achieve fluid isolation between the first and second ink tanks. Basically, this makes it possible to solve various problems attributable to the size of moving members as described above such as an ink tank and the rocking of a tube that have limited efforts to achieve a small size in the case of the continuous supply type.
When an intermittent supply type structure is adopted, however, it is important to discharge a gas such as air that enters or has entered an ink supply system and to control the pressure inside the same properly.
There are four general causes for the entrance of a gas into a supply system.
1) A gas can enter through ink ejection openings of a printing head or can generate as a result of an ejecting operation.
2) A gas that has been dissolved in ink can be separated from the same.
3) A gas can enter a supply system from the outside through the material from which the supply system is formed as a result of permeation.
4) A gas can enter when a joint is coupled to couple a first ink tank and a second ink tank.
The entrance of a gas is a problem that inevitably occurs, although the amount of the gas varies depending on the structure of the supply system. When a gas is accumulated in a second ink tank on a carriage for example, a problem arises in that the efficiency of charging the second ink tank with ink is reduced accordingly. Further, unexpected pressure changes are caused by expansion and contraction of air in response to temperature changes. This can result in leakage of ink from ejection openings attributable to an action of a resultant excessively large positive pressure or can conversely result in a failure of ink ejection attributable to an action of an excessively large negative pressure. Furthermore, the gas accumulated in the second ink tank can be included in ink that is guided to the ejection openings to cause problems such as disablement of ink ejection.
Such problems can be similarly caused when a continuous supply system of the tube supply type is configured. In a tube supply type continuous supply system in the related art, measures have been taken against such entrance of a gas, including a recovery operation for discharging ink and the gas from the printing head by simultaneously sucking them through the ejection openings thereof periodically or forcibly and a recovery operation performed in case that a second ink tank is carried by the carriage in which the gas is forcibly discharged from the second ink tank along with ink concurrently with an operation of sucking them through the ejection openings.
Since a great amount of waste ink is generated as a result of the adoption of such measures, serious limits are put on designing when a compact and portable printing apparatus is to be provided using the intermittent supply method. Further, a long time must be included in a control sequence of the printing apparatus to accommodate at least a recovery operation for sucking ink from the ejection openings of the printing head in addition to an operation of filling the second ink tank with ink at appropriate timing. In addition, since it is also required to perform a wiping operation for removing ink deposited on the surface of the printing head having the ejection openings formed thereon as a post-process for the recovery operation and a preliminary ejecting operation, a problem arises in that a further time is spent accordingly.
Referring to the continuous supply system of the tube supply type, in the case of an inkjet printing apparatus in which a pressure that is negative relative to the atmosphere must be generated to hold ink meniscuses formed at the ejection openings, there are limits including a need for providing the first ink tank in a position lower than the position of the ejection openings of the printing head in order to generate a negative pressure in the first ink tank naturally. This puts a limit on even the position and attitude or orientation of the ink tank and has resulted in problems including leakage of ink from the ejection openings especially in case that a portable printing apparatus is to be provided which is unstable in attitude during transportation.
On the contrary, proposals have been made for the adoption of the intermittent supply system, including a proposal in which a film having a function of allowing a gas to pass while disallowing a liquid to pass (hereinafter simply referred to as a functional film) is disposed to separately discharge only a gas from the second ink tank by force through the functional film and in which a porous member such as a sponge for holding ink is contained in the second ink tank to generate an adequate negative pressure therein. Such a structure is advantageous for even a portable printing apparatus whose attitude is unstable during transportation because it effectively suppresses an increase in the amount of waste ink generated during when ink is charged.
However, in order to use the functional film with stability, it is required that the film stays in a chemically inert state for a long time, which has resulted in a problem in that freedom in selecting ink is reduced, ink having a composition that does not affect the functional film must be selected.
When the functional film is provided on the second ink tank, a gas can conversely flow in the direction of entering the second ink tank. When a negative pressure generating mechanism such as a porous member for keeping ink under a negative pressure relative to a nozzle of the printing head is provided in the second ink tank for this reason, the efficiency of containing ink in the second ink tank is limited. Designing may be limited with respect to deposition of dyes and pigments in ink and endurance of the porous member against deterioration, which also reduces alternatives in selecting ink.
Further, in such a structure, since the porous member is always over-charged with ink when ink charging is completed, the over-charged ink in the porous member must be discharged as waste ink without fail by performing an operation of sucking the printing head through the ejection openings after the charging is completed in order to apply a required negative pressure to the printing head. That is, a problem arises in that a charging operation is accompanied by the generation of waste ink.
The invention was made taking the above problems into consideration, and it provides a structure in which an intermittent supply system is adopted as an ink supply system; high charging efficiency and a short charging time is achieved; and high overall usability of ink is achieved.
The invention thus contributes to the structure of a compact and portable inkjet printing apparatus.
In an aspect of the present invention, there is provided an inkjet printing apparatus for performing printing on a printing medium by using a printing head for ejecting ink, comprising:
a first ink tank serving as a source of the ink;
a second ink tank which can be charged with ink from the first ink tank and which supplies the ink to the printing head during printing; and
ink transport means for charging the second ink tank with the ink from the first ink tank,
wherein in relation to the direction of gravity in an orientation upon using the printing apparatus, a port of a channel located at a side of the second ink tank for supplying ink from the first ink tank to the second ink tank is arranged relatively high position and an ink outlet port for supplying ink from the second ink tank to the printing head is arranged relatively low position respectively, and a port of the channel located at a side of the first ink tank is arranged to position at the bottom of the first ink tank.
In another aspect of the present invention, there is provided an ink supplying method used for an inkjet printing apparatus for performing printing on a printing medium by using a printing head for ejecting ink, the method comprising the steps of;
providing an inkjet printing apparatus having:
a first ink tank serving as a source of the ink;
a second ink tank which can be charged with ink from the first ink tank and which supplies the ink to the printing head during printing; and
ink transport means for charging the second ink tank with the ink from the first ink tank,
wherein in relation to the direction of gravity in an orientation upon using the printing apparatus, a port of a channel located at a side of the second ink tank for supplying ink from the first ink tank to the second ink tank is arranged relatively high position and an ink outlet port for supplying ink from the second ink tank to the printing head is arranged relatively low position respectively, and a port of the channel located at a side of the first ink tank is arranged to position at the bottom of the first ink tank; and
wherein the ink transport means can return the contents of the second ink tank to the first ink tank, and in relation to the direction of gravity in an orientation upon using the printing apparatus, a starting point of a channel for the returning is in a relatively high position on the second ink tank in relation to the direction of gravity and a starting point of the channel for supplying ink to the printing head is in a relatively low position on the second ink tank in relation to the direction of gravity;
charging the second ink tank with the ink from the first ink tank by increasing the internal volume of the second ink tank; and
returning the contents of the second ink tank to the first ink tank by reducing the internal volume of the second ink tank.
According to the structure as set forth above, such an inkjet printing apparatus can be realized that the one comprises a first ink tank for supplying an ink and a second ink tank capable of charging ink at appropriate time from the first ink tank by an ink transfer means and for supplying the ink with the printing head upon the printing, wherein in an attitude or orientation upon using the printing apparatus a port of a channel located at a side of the second ink tank for supplying ink from the first ink tank to the second ink tank is arranged relatively high position and an ink outlet port for supplying ink from the second ink tank to the printing head is arranged relatively low position respectively in relation to the direction of gravity, and a port of the channel located at a side of the first ink tank is designed to position at the bottom of the first ink tank, thereby achieving a high charging efficiency to and shortened processing time for charging the second ink tank, and high overall usability of ink as well.
Moreover, due to the ink transfer means, the content of the second ink tank can be returned to the first ink tank appropriately prior to a charging operation. Also, because a point of the channel for starting the return is arranged at relatively high position on the second ink tank in an attitude or an orientation upon using the printing apparatus in relation to the direction of gravity, a gas residing within the second ink tank is transferred to the first ink tank securely and smoothly to achieve a smooth gas-liquid separation within the first ink tank. Such structure further enables avoiding gas thus returned to the first ink tank flowing into the second ink tank again.
Incidentally, in the present specification, the wording xe2x80x9cprintingxe2x80x9d means not only a condition of forming significant information such as characters and drawings, but also a condition of forming images, designs, patterns and the like on printing medium widely or a condition of processing the printing media, regardless of significance or unmeaning or of being actualized in such manner that a man can be perceptive through visual perception.
Further, the wording xe2x80x9cprinting mediumxe2x80x9d means not only a paper used in a conventional printing apparatus but also everything capable of accepting inks, such as fabrics, plastic films, metal plates, glasses, ceramics, wood and leathers, and in the following, will be also represented by a xe2x80x9csheetxe2x80x9d or simply by xe2x80x9cpaperxe2x80x9d.
Still further, the wording xe2x80x9cinkxe2x80x9d (also referred to as xe2x80x9cliquidxe2x80x9d in some occasions) should be interpreted in a broad sense as well as a definition of the above xe2x80x9cprintingxe2x80x9d and thus the ink, by being applied on the printing media, shall mean a liquid to be used for forming images, designs, patterns and the like, processing the printing medium or processing inks (for example, coagulation or encapsulation of coloring materials in the inks to be applied to the printing media).
Meantime, the present invention may be applied to a printing head in which a thermal energy generated by an electrothermal transducer is utilized to cause a film boiling to liquid in order to form bubbles, a printing head in which an electromechanical transducer is employed to eject liquid, a printing head in which a static electricity or air current is utilized to form and eject a liquid droplet and the others which are proposed in the art of an inkjet printing technology. Specifically, the printing head in which the electrothermal transducer is utilized is advantageously employed to achieve a compact structure.
Still further, the wording xe2x80x9cnozzlexe2x80x9d, as far as not mentioned specifically, represents to an ejection opening, a liquid passage communicated with the opening and an element for generating an energy used for ink, in summary.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.